1. Field of the Invention
The present invention relates to a fuel injection nozzle and more particularly to a variable injection hole type fuel injection nozzle.
2. Description of the Related Art
Extreme importance has been directed to NOx reduction in a low-speed, low-load region and to smoke reduction in the high-load region of high-pressure injection systems, though the high-pressure injection systems have been known effective in dealing with gas waste from diesel engines. In order to cope with the former problem, it is preferred to reduce the initial injection rate by effecting fuel injection for a long period of time using small-diameter injection holes and to establish optimum burning condition by accelerating fuel atomization, whereas in order to solve the latter problem, it is preferred to effect fuel injection for a short period of time using large-diameter injection holes.
However, a conventional fuel injection nozzle of the sort disclosed in Japanese Patent Unexamined Publication No. Sho 59-200063 has been only structured so that fuel is injected from an injection hole formed at the leading end of a nozzle body by forming a tapered pressure receiving face on the leading end side of a needle valve slidably accommodated in the nozzle body and letting the valve open because of fuel injection pressure. As a result, the injection hole diameter, that is, the injection hole area becomes fixed, which makes it impossible to deal with problems including expediting fuel burning, improving output.cndot.fuel cost, reducing not only noise resulting from fuel burning but also Nox and the like.
In order to tackle on the aforementioned problems, there has been proposed a variable injection nozzle designed for the injection hole area to be made variable and for the injection hole to be made switchable as desired by means of an actuator. An injection nozzle of such a type that has been proposed in Japanese Patent Unxamined Publication No. Hei 4-76266 has a plurality of injection holes circumferentially provided at predetermined intervals in the leading end portion of a nozzle body, the injection holes communicating with an internal hole to which a rotary valve is rotatably fitted, so that the opening of the injection hole is rendered adjustable as the rotary valve rotates.
The rotary valve type fuel injection nozzle like this is not designed to control injection holes at their axial positions as in a translation type fuel injection nozzle, that is, a fuel injection of such a type so as to move its valve shaft in the direction of an axial line. Thus, it is unnecessary to hold the position of injection holes against the axial force generated in the valve shaft due to the injection pressure and the pressure in the engine cylinder. Consequently, a desired injection hole area can be set by extremely small control torque if only the rotation of the rotary valve is controlled during the intake or exhaust stroke, whereby a very small actuator becomes usable with the merit of restraining the nozzle from becoming greater in size.
A specific injection nozzle according to the prior art, for example, includes a plurality (eight) of injection holes arranged in a circumferential hole wall, a rotary shaft provided as a rotary valve in the hole, and a plurality (four) of guide grooves circumferentially formed at predetermined intervals at the outer peripheral leading end of the rotary shaft, whereby the four and eight injection holes are selectively used for fuel injection as the rotary position of the rotary shaft varies.
Therefore, no injection hole variation using multiple injection holes can be set since the plurality of injection holes are situated only at one circumferential level. More specifically, it is impossible to control fuel injection by varying the injection hole diameter to deal with the waste gas as noted previously since the injection hole diameter itself remains invariable except that the number of injection holes having the same diameter on the same circumferential level can simply be increased or decreased according to the prior art. The problem is that atomization during the time a low load is applied is difficult to achieve.
In view of the fact that the diameter of the driving shaft of the rotary valve is small as the driving shaft is passed through the needle valve, moreover, the driving shaft is difficult to seal up. Therefore, fuel is caused to leak out of the driving shaft, which may result in lowering injection pressure or deficiency in fuel at the time of fuel injection.
In the case of a variable injection nozzle generally so constructed that injection holes are totally and temporarily closed when one injection hole diameter is switched to another, pressure in the nozzle body will sharply rise if one injection hole is switched to another during the time fuel injection is carried out. In case the needle valve ceases to operate for some reason or other or in case the follow-up opening of the rotary valve is delayed at the time the engine is operated at high speed, the pressure in the nozzle body will increase to the extent of danger in that the fuel injection system such as the fuel injection nozzles, the fuel injection pump or piping for connecting them is destroyed.